However, the windscreens initially manufactured to protect the driver or the pilot from the wind, bad weather, temperature differences or impacts with elements suspended in the air were by no means designed to allow for good quality image reflections. Whatever the projection mode used, collimated or not, the optical combiner must have a single reflection surface of good optical quality. However, the windscreens are made up of at least two dioptres, an input face and an output face that cause, when the light flux is reflected on the windscreen, a doubling of the observed image. This phenomenon becomes all the more pronounced with thicker windscreens, with poor parallel alignment between the dioptres and with curved windscreens. This phenomenon is illustrated in FIG. 1 which represents a viewing device comprising a display 1 which displays, for example, the number “123”. The two reflections on the dioptres 21 and 22 that form the windscreen 2 give two offset images of the number “123” represented in black and grey lines in FIG. 1. If the reflection coefficient of the external dioptre 22 is not negligible compared to that of the internal dioptre 21, these two offset images are seen by the user 3 and, obviously, the second image disturbs the perception of the first image. FIG. 2 represents the images generated IINIT and seen IVUE by a user after reflection on the windscreen, in the case of a white square on a black background. For reasons of clarity, the offset between the images reflected by the windscreen is chosen to be equal to one and a half times the side of the square so as to separate the different images without overlap. The top part of the FIG. 2 represents, on the right, the image generated on the display and, on the left, the variation of the light intensity IL along an axis x passing through the centre of the square. The bottom part of FIG. 2 represents, on the right, the image seen IVUE by the user and, on the left, the variation of the light intensity along an axis x passing through the centre of the image. The image of the square that is seen is doubled. The doubling of the image can be expressed in mathematical form. Thus, let IINIT(x,y) be an image generated on the display in a system of coordinates (x,y) and by denoting the image perceived by the user as IVUE, the images reflected by the dioptres of the windscreen as IREF1 and IREF2, the following relations apply:IREF1≈IINIT IREF2≈k.IINIT(x+dx,y+dy)IVUE≈IREF1+IREF2 
k is the relative reflection coefficient of the second image relative to the first image, k being strictly less than 1, dx and dy being the offsets introduced by the second dioptre of the windscreen.
This image doubling is now the major problem in viewing devices, also called HUD, for “head-up display”, that utilize projection onto a windscreen. The compensation methods are very costly and mostly require adaptation or even the use of dedicated windscreens, which, in most consumer applications, is a major obstacle.
Nowadays, in order to eliminate or strongly compensate the doubling of the image on the windscreen, the equipment manufacturers mainly use two different techniques which are:
use of a reflective film or “patch”.
The aim in this case is to prioritize a single reflection on the windscreen, that of the first dioptre, that is to say the internal face of the windscreen. This patch has to be completely parallel to the face on which it is deposited in order to ensure uniformity of reflection. The major drawback with these patches is chromatically filtering the transmission of the light flux, which is translated by a modified perception of the external colours. The outside scene is then darker in the area of the patch than in the rest of the field and major colour modifications may also be observed. Depending on the patch production techniques (thin-film deposition, etc.) the high cost may be a handicap to this technique.
a local modification of the geometry of the windscreen
Another technique involves modifying the structure of the windscreen. Windscreens are then obtained that are dedicated to the HUD function. The technique then lies in the modification of a plastic insert, made of “PVB” for example, in order to render the windscreen locally prismatic. The doubling of images is not eliminated, but it is made sufficiently great for the user to be able to perceive only a single image. The drawback with this solution is the need to produce a specific windscreen, dedicated to the HUD function and implement a complex and costly manufacturing method. This technical solution was envisaged notably by the company “Dupont” under the name of “Wedge”. This technique then requires windscreens to be replaced when the customer chooses the HUD option. This operation is particularly complex and costly to implement in the aeronautical field. Close collaboration is then needed between the designer of the projector and that of the windscreen in order to develop the system as a whole.